Image amplifier camera systems which are based on television or CCD cameras, storage film systems with an integrated or external readout unit, systems with optical coupling of a converter film to CCD cameras or CMOS chips, selenium-based detectors with electrostatic readout and X-ray detectors, in particular flat image detectors, having an active readout matrix with direct or indirect conversion of the X-radiation are known in digital X-ray imaging.
Such a detector is based on an active readout matrix, for example one or more detector plates made of amorphous silicon (a-Si) which is coated on the front with an X-ray converter layer or a scintillator layer. The active matrix is subdivided into a multiplicity of pixel readout units. In a so-called directly converting X-ray detector, the incident X-radiation is converted directly into electrical charge in the converter layer. In an indirectly converting X-ray detector, the incident X-radiation is converted into visible light in the scintillator and then in turn converted into electrical charge in photodiodes of the active matrix. This charge can then be stored in the pixel readout units and read out; the resulting raw X-ray images may subsequently be post-processed.
In order to obtain high-quality X-ray images, some of the effects due to the specific properties of the respective X-ray detector need to be corrected electronically. The most important electronic corrections are offset corrections in which the dark current is corrected, and gain corrections which compensate for sensitivity variations of the X-ray detector. In general offset corrections are carried out by electronic subtraction, and gain corrections by electronic multiplication with previously compiled offset or gain calibration images.
Overall the combined offset and gain correction, the so-called flat field correction, can generally be described in the following way:K=G·[S−O] or Ki=Gi·[Si−Oi],where S represents the raw X-ray image and Si the raw value relating to the respective pixel readout element i, O represents the offset correction image and Oi the offset value, G represents the gain correction image and Gi the gain value, K represents the corrected X-ray image and Ki the corrected final value.
Temperature changes of the X-ray detector generally lead to offset structures and sensitivity differences, above all in edge and transition regions of the active matrix of the X-ray converter. The latter may, for example, occur by temperature-related expansion or contraction of layers. If calibration images are recorded at a different temperature than the raw X-ray images, then false corrections often occur.
It is known from DE 10 2004 003 881 A1 to monitor the temperature of an X-ray detector and record a new calibration image in the event of a substantial temperature rise of the X-ray detector.
It is however often not possible to record a calibration image matching each X-ray recording, in particular a gain calibration image, rather it is necessary to resort to gain calibration images already recorded. These have generally been recorded at temperatures differing from the exact recording temperature.